Advances in cancer treatment have been hampered by a limited understanding of the mechanisms blocking apoptosis that is mediated by death receptors such as Fas/CD95/Apo-1. It is surprising that there is little research directed toward restoring Fas receptor, despite its pervasiveness in cancer and possible beneficial role in cancer therapy. Restoring Fas-apoptosis to cancer cells would be a major breakthrough in cancer therapy. We screened non-Hodgkin lymphoma (NHL) cells for inhibitors of Fas and identified CD74 as a candidate. CD74 is a major histocompatibility complex-associated protein that is highly expressed in hematopoietic cancers. We showed that CD74 binds Fas and suppresses Fas-mediated apoptosis. We also showed that human chronic lymphocytic leukemia (CLL) and NHL tumor tissues contain complexes of CD74-Fas. We disrupted the CD74- Fas complex with competing peptides and with an anti-CD74 antibody, which substantially facilitated Fas- mediated apoptosis. In a clinical trial we show anti-CD74 antibody therapy is associated with disruption of CD74-Fas complexes. We therefore hypothesize that CD74-Fas complexes inhibit apoptosis and can be disrupted to enhance apoptosis in vivo. In a clinical trial using anti-CD74 antibody for patients with CLL and NHL, we will correlate CD74 antibody therapy with intercellular mediators of apoptosis and CD74-dependent signaling. We will also analyze plasma before and during chemotherapy for intercellular CD74-Fas-related signaling markers. We will identify the predominant intracellular signaling pathway activated in antitumor responses with CD74-targeted therapy. As an alternative plan, we will analyze CD74-Fas signaling in CLL cells from patients before and during therapy with fludarabine, cyclophosphamide, rituximab, which uses Fas- mediated apoptosis in tumor regression. The long-term goal of this project is to develop a detailed understanding of mechanisms by which inhibitors of Fas can be modulated to enhance cancer cell apoptosis. PUBLIC HEALTH RELEVANCE: Lymphoma and leukemia express Fas but are commonly resistant to Fas-mediated apoptosis. We have identified an inhibitor of Fas, termed CD74, and will treat patients with the anti-CD74 antibody. We will determine if CD74 antibodies sensitize cancer cells to apoptosis in vivo by examining CD74-dependent signaling and apoptosis rates.